Field of the Invention
The present invention relates to a capacitive transducer to be used as an ultrasonic transducer or the like and a capacitive transducer manufacturing method.
Description of the Related Art
Ultrasonic transducers, which are designed to transmit and receive an ultrasonic wave, are used in a diagnosis apparatus for making a diagnosis of tumor inside a living body, for example. In recent years, a capacitive transducer using micromachining technology (capacitive micromachined ultrasonic transducer: CMUT) is being developed. The CMUT is superior to a conventional ultrasonic transducer using a piezoelectric element in obtaining broadband characteristics easily and being low in noise with a less number of vibration modes. The feature of the CMUT is to transmit or receive an acoustic wave, such as an ultrasonic wave, a sonic wave, and a photoacoustic wave (hereinafter sometimes represented by ultrasonic wave) with the use of a lightweight vibrating film. Attention is being paid on this promising technology of using the CMUT to make an accurate ultrasonic diagnosis as compared to a conventional medical diagnostic modality.
As the above-mentioned technology, a capacitive transducer that realizes broadband characteristics (characteristics with a relatively high electromechanical transformer ratio in a wide frequency domain) has been proposed, which includes a cell including a vibrating film having a high spring constant and a cell including a vibrating film having a low spring constant (see U.S. Pat. No. 5,870,351). Another capacitive transducer that realizes broadband characteristics has been proposed, which has a cell group of multiple cells having a high spring constant and a cell group of multiple cells having a low spring constant (see US 2007/0059858).
In the capacitive transducers as described above, the broadband is attained by using multiple cells including multiple vibrating films having different spring constants. Thus, the capacitive transducer typically includes multiple cells including multiple vibrating films having different areas.
The capacitive transducer is manufactured by using, for example, surface micromachining technology, mainly sacrificial layer etching technology. In the surface micromachining technology, however, a thin film portion to be shaped later into a vibrating film may adhere onto a lower surface of a gap formed by etching the sacrificial layer in a drying step following the etching of the sacrificial layer. Even when the capacitive transducer can be manufactured, the vibrating film is liable to be warped by stress, and, in particular, a vibrating film having a large area is liable to be warped because the spring constant is low. Depending on the stress distribution of the vibrating film, the vibrating film is unevenly warped to deteriorate the performance. The challenge to realize a capacitive transducer having broadband characteristics with a high electromechanical transformer ratio is to form a device by manufacturing multiple cells having different frequency characteristics of the electromechanical transformer ratios uniformly in a single element.